1. Field of the Invention
The present invention relates to an electron beam apparatus.
2. Description of Related Art
In order to perform inspection or observation of the shape of a fine pattern electronic device, conventionally various electron beam devices, such as a scanning electron microscope, have been used. In particular, there has been a high demand for high resolution observation accompanying the fact that electronic devices have become ultrafine in recent years. With this type of electron beam device capable of high-resolution observation, there are cases where a potential is applied to a sample in order to increase image quality at low accelerating voltages. Therefore, when a potential Vs is applied to a sample, secondary electrons then have substantially the energy Vs and the proportion of secondary electrons on the electron source side traveling in a straight line along the optical axis is therefore high. As a result, there is a problem where the efficiency with which secondary electrons are detected falls. This tendency is therefore particularly strong for electrons coming from the bottoms of holes in a semiconductor wafer.
In order to resolve this problem, in Japanese Laid-open Patent Publication No. Hei. 5-258703, there is disclosed an electron beam scanning method and system thereof that deflects secondary electrons using a Wien filter so as to detect secondary electrons while reducing the effect on incident electrons. At the Wien filter, the electric field and the magnetic field are arranged so as to be orthogonal with respect to each other. Of the force acting on electrons incident in a direction from the electron gun towards the sample, force due to the electric field is −eE, and force due to the magnetic field is −e(v×B) constituting the vector sum. The forces acting on the incident electrons when these forces are equal are therefore balanced and incident electrons are therefore not deflected. However, the direction of travel of secondary electrons coming from the sample is opposite to the direction of incident electrons with respect to the Wien filter. The direction of force due to the magnetic field is therefore opposite to that of the incident electrons. The forces of the electric field and the magnetic field therefore act on the secondary electrons in the same direction and therefore only the secondary electrons are deflected.
In addition to the methods proposed in Japanese Laid-open Patent Publication Hei. 5-258703, methods are proposed in Japanese Patent Publication No. 3136353 and Japanese Laid-open Patent publication No. Hei. 10-214586 where a plurality of secondary electron detectors are provided along the direction of the optical axis, with the sum of a plurality of detector signals then being used to improve the efficiency with which secondary electrons are detected.
However, in the invention disclosed in Japanese Laid-open Patent Publication Hei. 5-258703, the secondary electron detector is installed away from the beam that is on the optical axis. It is therefore necessary to deflect secondary electrons through an angle greater than that of a detection surface of a following scattered electron detector using two deflectors functioning as a Wien filter in order to detect the secondary electrons. The intensity of the electromagnetic field of the Wien filter is therefore large, and the influence of Wien filter aberrations on the beam therefore easily becomes large which can easily prove detrimental to image resolution.
Further, a large apparatus capable of generating a large electromagnetic field for the deflectors functioning as the Wien filter is required in order to deflect the secondary electrons through a large angle. In addition to making the apparatus manufacturing costs high, this also makes the electron beam apparatus as a whole large, which results in an expensive product.
With the invention disclosed in Japanese Patent No. 3136353, the secondary electrons pass through the hole in the upper side detector meaning that there are secondary electrons that are not detected at the upper side detector. This means that detection efficiency is not sufficient. Further, a deflector is provided in order to perform beam scanning on the sample surface but the amount of secondary electrons deflected depends on the strength of the deflector and there is therefore a further problem in that it is easy for the center of the screen to become dark.
In the invention proposed in Japanese Laid-open Patent Publication No. 10-214586, the electron-gun-side axis and the objective lens axis are separated just by a prescribed distance. When this prescribed distance is fixed, the intensity of the beam deflector is fixed because the electron gun axis and the objective lens axis are in line with each other. It has therefore been difficult to set the strength of the deflectors appropriately so as to obtain a sufficient secondary electron signal. There is also a further problem of inconvenience in that this is detrimental to flexibility during lens barrel adjustment. On the other hand, a mechanism for, for example, moving the electron gun axis horizontally etc. is required when it is wished to vary a prescribed distance between the electron gun-side axis and the objective lens axis. This makes the structure of the electron beam apparatus complex, causes the manufacturing cost of the apparatus to rise, and makes the operation of the electron beam apparatus complex.
A secondary electron detector having a hole allowing a beam to pass is used as a secondary electron detector and a Micro Channel Plate (MCP) is given as this kind of detector. However, MCPs have a short lifespan compared to the scintillator/photo-multiplier detectors usually used and are therefore expensive. In the case where a secondary detector having a hole allowing this beam to pass is used as the second detector, the secondary electrons are incident to the detection surface in the shape of a spot and there is the fear that this will cause the detection surface to deteriorate rapidly.
Further, a secondary electron detector having a hole allowing a beam to pass encompasses the detection electrode to which the negative potential is applied at the inside of the hole. There are therefore parts between the detection surface and the electrode where detection cannot be achieved. In particular, it is necessary to make the strength of the beam deflector substantial in order to surpass portions that cannot detect and deflect the beam as far as the detection surface. There is therefore the problem that deflection aberrations are increased and image resolution is caused to deteriorate.